Electric machines

ABSTRACT

Stators for radial-flux rotary electric machines and methods of assembling stators for radial flux rotary electric machines are provided. One such stator comprises: a yoke with a plurality of teeth, each tooth extending from a root at the yoke to a tip, the width of each tooth at the root being greater than or equal to the width at the tip thereof to define open slots therebetween; a plurality of coils, each one of which is located on a respective one of the plurality of teeth, wherein the coils are sized such that the slot fill factor of between zero and unity to define axial voids in the slots; and a plurality of magnetic sealing members, each one of which is engaged with the tips of adjacent teeth to circumferentially and axially seal the axial voids in the slots for retaining an axial flow of coolant therein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority fromUK Patent Application Number 1913083.0 filed on 11 Sep. 2019, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

This disclosure concerns stators for a radial-flux rotary electricmachines.

BACKGROUND

In aerospace, the more electric engine (MEE) and more electric aircraft(MEA) concepts have been advocated for the substantial reduction in fuelconsumption and complexity they bring. Service experience has shownhowever that the means of electrical generation in such configurationscan and do fail from time to time. In addition, the electrical aspectsof the devices are considered complex systems and so a rate approach tocertification is not possible. This imposes a requirement forfault-tolerant designs. It is accepted practice to provide singlefault-tolerance, on the basis that the mean time before failure for thesurviving system is sufficiently long.

In one known approach, two individual starter-generator devices areinstalled per engine, each one of which is connected at a respective padon the accessory gearbox. In the event that one device fails, the othermay take over the full duty. This approach however has, duringfault-free operation, an attendant 100 percent over-rating in terms ofpower, weight, and installation volume simply to meet the requirement ofsingle fault-tolerance.

Other approaches that have been proposed involve packaging twostarter-generators with a common housing such that they share a commonshaft, and utilise only one pad on an accessory gearbox. This may beachieved as the common mechanical parts may be engineered to have asufficiently low failure rate that the likelihood of failure isextremely remote, with duplexing of the electrical components providingthe single fault-tolerance. Whilst this reduces the installationcomplexity and to a degree reduces weight, there is still a 100 percentoverrating in terms of electrical generation capacity.

Whilst at a smaller scale, namely for an aircraft fuel pump, the presentapplicant has developed independent four-phase drive systems. The use ofindependent phase drives permits a single phase to develop a fault, withthe remaining three phases providing continued operation. In suchconfigurations the degree of overrating of capacity may be limited to 33percent, thus resulting in a substantial weight saving in the electricmachine, albeit at the expense of a more complex drive system.

It is an object of the invention to apply such a four-phase approach ina future MEA-compatible system.

It is an object of the invention to apply such a four-phase approach toa motor-generator suitable for starting a gas turbine engine andproviding power to an aircraft.

It is an object of the invention to provide a cooling scheme for such amotor-generator.

It is an object of the invention to reduce the complexity of assembly ofsuch a machine.

SUMMARY

The invention is directed towards stators for radial-flux rotaryelectric machines and methods for assembling the same.

One such stator comprises:

a yoke with a plurality of teeth, each tooth extending from a root atthe yoke to a tip, the width of each tooth at the root being greaterthan or equal to the width at the tip thereof to define open slotstherebetween;

a plurality of coils, each one of which is located on a respective oneof the plurality of teeth, wherein the coils are sized such that theslot fill factor of between zero and unity to define axial voids in theslots;

a plurality of magnetic sealing members, each one of which is engagedwith the tips of adjacent teeth to circumferentially and axially sealthe axial voids in the slots for retaining an axial flow of coolanttherein.

In an embodiment, the stator is an alternate-wound stator having anumber n≥4 of teeth and n/2 coils, and in which each slot is defined bya wound tooth carrying a coil, and an unwound tooth not carrying a coil.

In an embodiment, the width of wound teeth is greater than the width ofunwound teeth.

In an embodiment, the width of wound teeth is twice the width of unwoundteeth.

In an embodiment, the stator has 16 slots and four coil pairs, each coilpair forming part of one of four independent electrical phases.

In an embodiment, each coil pair comprises coils that are 180 mechanicaldegrees apart.

In an embodiment, the tip of each tooth includes axial grooves forreceiving axial ribs on the sealing members.

In an embodiment, the coils are formed so as to have a substantiallyparallelogram-shaped cross section, such that the axial voids are oftriangular cross section.

In an embodiment, the stator has a slot fill-factor of 0.22 to 0.28.

In an embodiment, the ratio of the stator inner diameter to the statorouter diameter is from 0.6 to 0.7.

In an embodiment, the ratio of the stator inner diameter to the statorouter diameter is from 0.63 to 0.67.

In an embodiment, each coil has a per-unit inductance of from 1.1 to1.4.

In an embodiment, a saturation flux density of the sealing members isdifferent to a saturation flux density of the yoke.

A method of assembling a stator for a radial flux rotary electricmachine is also provided, and comprises:

obtaining a stator yoke with a plurality of teeth, each of which extendsfrom a root at the yoke to a tip, the width of each tooth at the rootbeing greater than or equal to the width at the tip thereof to defineopen slots between adjacent teeth;

for each one at least a portion of the teeth, placing a coil on a tooth,wherein the coil is sized such that the slot fill factor of between zeroand unity to define an axial void in the slots either side of the tooth;

for each slot, engaging a magnetic sealing member with the tips of theadjacent teeth to circumferentially and axially seal the axial void inthe slot for retaining an axial flow of coolant therein.

In an embodiment, the method further comprises a vacuum pressureimpregnation procedure after the engaging step to seal the sealingmembers to the stator yoke.

In an embodiment, the method further comprises forming the coilsoff-iron prior to placing them on the stator yoke.

In an embodiment, the placing step comprises winding the coils on thestator yoke.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with referenceto the accompanying drawings, which are purely schematic and

not to scale, and in which:

FIG. 1 shows a general arrangement of a turbofan engine for an aircraft,including a motor-generator and two dc busses for the aircraft;

FIG. 2 shows an electric drive system for the motor-generator of FIG. 1;

FIG. 3 is schematic of the winding scheme of the motor-generator of FIG.1;

FIG. 4 is a radial cross-section of the motor-generator on I-I of FIG.5;

FIG. 5 is an axial cross-section of the motor-generator on II-II of FIG.4;

FIG. 6 is a magnified view of the stator of the motor-generator;

FIG. 7 is a schematic of coolant flow in the motor-generator;

FIG. 8A is an end-on view of the inner rotor component of the rotor ofthe motor-generator;

FIG. 8B is a side view of the inner rotor component of the rotor of themotor-generator;

FIG. 8C is a developed plan view of the stator cooling path at thearcuate section of FIG. 4.

FIG. 9A is another magnified view of the stator of the motor-generator;

FIG. 9B is an isometric view of the sealing elements in the stator ofthe motor-generator;

FIG. 10 shows steps carried out by the controller of the electric drivesystem of FIG. 2;

FIG. 11 shows steps carried out by the controller to respond to a phasefault;

FIG. 12 shows steps carried out by the controller to respond to a busfault.

DETAILED DESCRIPTION

FIG. 1

A general arrangement of an engine 101 for an aircraft is shown inFIG. 1. In the present embodiment, the engine 101 is of turbofanconfiguration, and thus comprises a ducted fan 102 that receives intakeair A and generates two pressurised airflows: a bypass flow B whichpasses axially through a bypass duct 103 and a core flow C which entersa core gas turbine.

The core gas turbine comprises, in axial flow series, a low-pressurecompressor 104, a high-pressure compressor 105, a combustor 106, ahigh-pressure turbine 107, and a low-pressure turbine 108.

In use, the core flow C is compressed by the low-pressure compressor 104and is then directed into the high-pressure compressor 105 where furthercompression takes place. The compressed air exhausted from thehigh-pressure compressor 105 is directed into the combustor 106 where itis mixed with fuel and the mixture is combusted. The resultant hotcombustion products then expand through, and thereby drive, thehigh-pressure turbine 107 and in turn the low-pressure turbine 108before being exhausted to provide a small proportion of the overallthrust.

The high-pressure turbine 107 drives the high-pressure compressor 105via an interconnecting shaft. The low-pressure turbine 108 drives thelow-pressure compressor 104 via another interconnecting shaft. Together,the high-pressure compressor 105, high-pressure turbine 107, andassociated interconnecting shaft form part of a high-pressure spool ofthe engine 101. Similarly, the low-pressure compressor 104, low-pressureturbine 108, and associated interconnecting shaft form part of alow-pressure spool of the engine 101. Such nomenclature will be familiarto those skilled in the art.

The fan 102 is driven by the low-pressure turbine 108 via a reductiongearbox in the form of a planetary-configuration epicyclic gearbox 109.Thus in this configuration, the low-pressure turbine 108 is connectedwith a sun gear of the gearbox 109. The sun gear is meshed with aplurality of planet gears located in a rotating carrier, which planetgears are in turn are meshed with a static ring gear. The rotatingcarrier drives the fan 102 via a fan shaft 110.

It will be appreciated that in alternative embodiments astar-configuration epicyclic gearbox (in which the planet carrier isstatic and the ring gear rotates and provides the output) may be usedinstead.

As described previously, it is desirable to implement a greater degreeof electrical functionality on the airframe and on the engine. To thisend, the engine 101 comprises a high-pressure spool driven, core-mountedaccessory gearbox 111 of conventional drive configuration, and which hasa motor-generator 112 mounted thereto. As well as operation as agenerator to supply an aircraft on which the engine 101 is installedwith electrical power, the motor-generator 112 may drive thehigh-pressure spool to facilitate starting of the engine 101 in place ofan air turbine starter.

It will of course be appreciated by those skilled in the art that anysuitable location for the motor-generator 112 may be adopted. Forexample, the motor-generator 112 may be mounted on the engine centrelineaxially forward of the high-pressure compressor 105, directly connectedwith the high-pressure spool.

In the present example, the motor-generator 112 operates in response tocommand signals received from an engine electronic controller (EEC) 113,which in turn responds to demand signals received from the aircraft onwhich the engine is installed. In the present embodiment the EEC 113 isa full-authority digital engine controller (FADEC), the configuration ofwhich will be known and understood by those skilled in the art.

In an implementation contemplated herein, the motor-generator 112 isconfigured such that it may output to or receive electrical power fromtwo dc busses—a configuration contemplated for future more electricaircraft platforms. The configuration of this electric drive system willbe described with reference to FIG. 2. It will be appreciated by thoseskilled in the art however that in alternative implementationselectrical power may be provided to the motor-generator 112 by way of analternating current supply, for example from an external supply during astarting procedure for the engine 101.

The fault-tolerant radial flux configuration of the motor-generator 112will be described with reference to FIGS. 3 to 6.

The cooling system for the motor-generator 112 will be described withreference to FIGS. 7 through 9B.

Control strategies to respond to different faults in the motor-generator112 or the broader electric drive system will be described withreference to FIGS. 10 to 12.

Various embodiments of the engine 101 may include one or more of thefollowing features.

It will be appreciated that instead of being a turbofan having a ductedfan arrangement, the engine 101 may instead be a turboprop comprising apropeller for producing thrust.

The low- and high-pressure compressors 104 and 105 may comprise anynumber of stages, for example multiple stages. Each stage may comprise arow of rotor blades and a row of stator vanes, which may be variablestator vanes (in that their angle of incidence may be variable). Inaddition to, or in place of, axial stages, the low- or high-pressurecompressors 104 and 105 may comprise centrifugal compression stages.

The low- and high-pressure turbines 107 and 108 may also comprise anynumber of stages.

The fan 102 may have any desired number of fan blades, for example 16,18, 20, or 22 fan blades.

Each fan blade may be defined as having a radial span extending from aroot (or hub) at a radially inner gas-washed location, or 0 percent spanposition, to a tip at a 100 percent span position. The ratio of theradius of the fan blade at the hub to the radius of the fan blade at thetip—the hub-tip ratio—may be less than (or on the order of) any of: 0.4,0.39, 0.38 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28,0.27, 0.26, or 0.25. The hub-tip ratio may be in an inclusive rangebounded by any two of the aforesaid values (i.e. the values may formupper or lower bounds). The hub-tip ratio may both be measured at theleading edge (or axially forwardmost) part of the blade. The hub-tipratio refers, of course, to the gas-washed portion of the fan blade,i.e. the portion radially outside any platform.

The radius of the fan 102 may be measured between the engine centrelineand the tip of a fan blade at its leading edge. The fan diameter may begreater than (or on the order of) any of: 2.5 metres, 2.6 metres, 2.7metres, 2.8 metres, 2.9 metres, 3 metres, 3.1 metres, 3.2 metres, 3.3metres, 3.4 metres, 3.5 metres, 3.6 metres, 3.7 metres, 3.8 metres, or3.9 metres.

The fan diameter may be in an inclusive range bounded by any two of theaforesaid values (i.e. the values may form upper or lower bounds).

The rotational speed of the fan 102 may vary in use. Generally, therotational speed is lower for fans with a higher diameter. Purely by wayof non-limitative example, the rotational speed of the fan at cruiseconditions may be less than 2500 rpm, for example less than 2300 rpm.Purely by way of further non-limitative example, the rotational speed ofthe fan 102 at cruise conditions for an engine having a fan diameter inthe range of from 2.5 metres to 3 metres (for example 2.5 metres to 2.8metres) may be in the range of from 1700 rpm to 2500 rpm, for example inthe range of from 1800 rpm to 2300 rpm, or, for example in the range offrom 1900 rpm to 2100 rpm. Purely by way of further non-limitativeexample, the rotational speed of the fan at cruise conditions for anengine having a fan diameter in the range of from 3.2 metres to 3.8metres may be in the range of from 1200 rpm to 2000 rpm, for example inthe range of from 1300 rpm to 1800 rpm, for example in the range of from1400 rpm to 1600 rpm.

In use of the engine 101, the fan 102 (with its associated fan blades)rotates about a rotational axis. This rotation results in the tip of thefan blade moving with a velocity U_(tip). The work done by the fanblades on the flow results in an enthalpy rise dH of the flow. A fan tiploading may be defined as dH/U_(tip) ², where dH is the enthalpy rise(for example the one dimensional average enthalpy rise) across the fanand U_(tip) is the (translational) velocity of the fan tip, for exampleat the leading edge of the tip (which may be defined as fan tip radiusat leading edge multiplied by angular speed). The fan tip loading atcruise conditions may be greater than (or on the order of) any of: 0.3,0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 or 0.4. The fan tiploading may be in an inclusive range bounded by any two of the values inthe previous sentence (i.e. the values may form upper or lower bounds).

The engine 101 may have any desired bypass ratio, where the bypass ratiois defined as the ratio of the mass flow rate of the flow B through thebypass duct to the mass flow rate of the flow C through the core atcruise conditions. Depending upon the selected configuration, the bypassratio may be greater than (or on the order of) any of the following: 10,10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, or 17.The bypass ratio may be in an inclusive range bounded by any two of theaforesaid values (i.e. the values may form upper or lower bounds). Thebypass duct may be substantially annular. The bypass duct may beradially outside the core engine. The radially outer surface of thebypass duct may be defined by a nacelle and/or a fan case.

The overall pressure ratio of the engine 101 may be defined as the ratioof the stagnation pressure upstream of the fan 102 to the stagnationpressure at the exit of the high-pressure compressor 105 (before entryinto the combustor). By way of non-limitative example, the overallpressure ratio of the engine 101 at cruise may be greater than (or onthe order of) any of the following: 35, 40, 45, 50, 55, 60, 65, 70, 75.The overall pressure ratio may be in an inclusive range bounded by anytwo of the aforesaid values (i.e. the values may form upper or lowerbounds).

Specific thrust of the engine 101 may be defined as the net thrust ofthe engine divided by the total mass flow through the engine 101. Atcruise conditions, the specific thrust of the engine 101 may be lessthan (or on the order of) any of the following: 110 Nkg⁻¹ s, 105 Nkg⁻¹s, 100 Nkg⁻¹ s, 95 Nkg⁻¹ s, 90 Nkg⁻¹ s, 85 Nkg⁻¹ s, or 80 Nkg⁻¹ s. Thespecific thrust may be in an inclusive range bounded by any two of thevalues in the previous sentence (i.e. the values may form upper or lowerbounds). Such engines may be particularly efficient in comparison withconventional gas turbine engines.

The engine 101 may have any desired maximum thrust. For example, theengine 101 may be capable of producing a maximum thrust of at least (oron the order of) any of the following: 160 kilonewtons, 170 kilonewtons,180 kilonewtons, 190 kilonewtons, 200 kilonewtons, 250 kilonewtons, 300kilonewtons, 350 kilonewtons, 400 kilonewtons, 450 kilonewtons, 500kilonewtons, or 550 kilonewtons. The maximum thrust may be in aninclusive range bounded by any two of the aforesaid values (i.e. thevalues may form upper or lower bounds). The thrust referred to above maybe the maximum net thrust at standard atmospheric conditions at sealevel plus 15 degrees Celsius (ambient pressure 101.3 kilopascals,temperature 30 degrees Celsius), with the engine 101 being static.

In use, the temperature of the flow at the entry to the high-pressureturbine 107 may be particularly high. This temperature, which may bereferred to as turbine entry temperature or TET, may be measured at theexit to the combustor 106, for example immediately upstream of the firstturbine vane, which itself may be referred to as a nozzle guide vane. Atcruise, the TET may be at least (or on the order of) any of thefollowing: 1400 kelvin, 1450 kelvin, 1500 kelvin, 1550 kelvin, 1600kelvin or 1650 kelvin. The TET at cruise may be in an inclusive rangebounded by any two of the aforesaid values (i.e. the values may formupper or lower bounds). The maximum TET in use of the engine 101 may be,for example, at least (or on the order of) any of the following: 1700kelvin, 1750 kelvin, 1800 kelvin, 1850 kelvin, 1900 kelvin, 1950 kelvinor 2000 kelvin. The maximum TET may be in an inclusive range bounded byany two of the aforesaid values (i.e. the values may form upper or lowerbounds). The maximum TET may occur, for example, at a high thrustcondition, for example at a maximum take-off (MTO) condition.

A fan blade and/or aerofoil portion of a fan blade described and/orclaimed herein may be manufactured from any suitable material orcombination of materials. For example at least a part of the fan bladeand/or aerofoil may be manufactured at least in part from a composite,for example a metal matrix composite and/or an organic matrix composite,such as carbon fibre. By way of further example at least a part of thefan blade and/or aerofoil may be manufactured at least in part from ametal, such as a titanium based metal or an aluminium based material(such as an aluminium-lithium alloy) or a steel based material. The fanblade may comprise at least two regions manufactured using differentmaterials. For example, the fan blade may have a protective leadingedge, which may be manufactured using a material that is better able toresist impact (for example from birds, ice or other material) than therest of the blade. Such a leading edge may, for example, be manufacturedusing titanium or a titanium-based alloy. Thus, purely by way ofexample, the fan blade may have a carbon-fibre or aluminium-based bodywith a titanium leading edge.

The fan 102 may comprise a central hub portion, from which the fanblades may extend, for example in a radial direction. The fan blades maybe attached to the central portion in any desired manner. For example,each fan blade may comprise a fixture which may engage a correspondingslot in the hub. Purely by way of example, such a fixture may be in theform of a dovetail that may slot into and/or engage a corresponding slotin the hub/disc in order to fix the fan blade to the hub. By way offurther example, the fan blades maybe formed integrally with a centralhub portion. Such an arrangement may be a bladed disc or a bladed ring.Any suitable method may be used to manufacture such a bladed disc orbladed ring. For example, at least a part of the fan blades may bemachined from a billet and/or at least part of the fan blades may beattached to the hub/disc by welding, such as linear friction welding.

The engine 101 may be provided with a variable area nozzle (VAN). Such avariable area nozzle may allow the exit area of the bypass duct to bevaried in use. The general principles of the present disclosure mayapply to engines with or without a VAN.

As used herein, cruise conditions have the conventional meaning andwould be readily understood by the skilled person. Thus, for a given gasturbine engine for an aircraft, the skilled person would immediatelyrecognise cruise conditions to mean the operating point of the engine atmid-cruise of a given mission (which may be referred to in the art asthe “economic mission”) may mean cruise conditions of an aircraft towhich the gas turbine engine is designed to be attached. In this regard,mid-cruise is the point in an aircraft flight cycle at which 50 percentof the total fuel that is burned between top of climb and start ofdescent has been burned (which may be approximated by the midpoint—Suchcruise conditions may be conventionally defined as the conditions atmid-cruise, for example the conditions experienced by the aircraftand/or engine at the midpoint (in terms of time and/or distance-)between top of climb and start of descent. Cruise conditions thus definean operating point of, the gas turbine engine that provides a thrustthat would ensure steady state operation (i.e. maintaining a constantaltitude and constant Mach number) at mid-cruise of an aircraft to whichit is designed to be attached, taking into account the number of enginesprovided to that aircraft. For example where an engine is designed to beattached to an aircraft that has two engines of the same type, at cruiseconditions the engine provides half of the total thrust that would berequired for steady state operation of that aircraft at mid-cruise.

In other words, for a given gas turbine engine for an aircraft, cruiseconditions are defined as the operating point of the engine thatprovides a specified thrust (required to provide—in combination with anyother engines on the aircraft—steady state operation of the aircraft towhich it is designed to be attached at a given mid-cruise Mach Number)at the mid-cruise atmospheric conditions (defined by the InternationalStandard Atmosphere according to ISO 2533 at the mid-cruise altitude).For any given gas turbine engine for an aircraft, the mid-cruise thrust,atmospheric conditions and Mach number are known, and thus the operatingpoint of the engine at cruise conditions is clearly defined.

The cruise conditions may correspond to ISA standard atmosphericconditions at an altitude that is in the range of from 10000 to 15000metres, such as from 10000 to 12000 metres, or from 10400 to 11600metres (around 38000 feet), or from 10500 to 11500 metres, or from 10600to 11400 metres, or from 10700 metres (around 35000 feet) to 11300metres, or from 10800 to 11200 metres, or from 10900 to 11100 metres, or11000 metres. The cruise conditions may correspond to standardatmospheric conditions at any given altitude in these ranges.

The forward speed at the cruise condition may be any point in the rangeof from Mach 0.7 to 0.9, for example one of Mach 0.75 to 0.85, Mach 0.76to 0.84, Mach 0.77 to 0.83, Mach 0.78 to 0.82, Mach 0.79 to 0.81, Mach0.8, Mach 0.85, or in the range of from Mach 0.8 to 0.85. Any singlespeed within these ranges may be the cruise condition. For someaircraft, the cruise conditions may be outside these ranges, for examplebelow Mach 0.7 or above Mach 0.9.

Thus, for example, the cruise conditions may correspond specifically toa pressure of 23 kilopascals, a temperature of minus 55 degrees Celsius,and a forward Mach number of 0.8.

It will of course be appreciated, however, that the principles of theinvention claimed herein may still be applied to engines having suitabledesign features falling outside of the aforesaid parameter ranges.

FIG. 2

As described previously, in an aspect the motor-generator 112 isconfigured such that it may output to or receive electrical power fromtwo dc busses in a more electric aircraft installation. Such a system,hereinafter referred to as an electric drive system, is shown in FIG. 2in the form of a logical single-line diagram.

The motor-generator 112 comprises four phase connections, ϕA, ϕB, ϕC,and ϕD, which are each connected with an ac side of a respectiveindependent phase drive circuit 201, 202, 203, and 204. As will bedescribed with reference to FIG. 3, in the present embodiment theinternal topology of the motor-generator 112 is of duplex four phaseconfiguration, and thus in practice an additional set of connections maybe provided to provide parallel connection. Alternatively connection maybe made in series either internally to the motor-generator 112 orexternally.

Referring again to FIG. 2, in the present embodiment the phase drivecircuits 201 to 204 are bidirectional converter circuits. In a specificembodiment, the phase drive circuits 201 to 204 are H-bridgesaccompanied by appropriate filters, although it will be appreciated thatany other suitable bidirectional converter topology may be used, such asa neutral point clamped converter topology or a pseudo-resonantsoft-switching full bridge topology.

The phase drive circuits 201 to 204 operate under control of acontroller 205, which co-ordinates the operation of the phase drivecircuits 201 to 204 to effect rectification or inversion as appropriate.

In the present embodiment, the controller 205 in turn operates undercontrol of the EEC 113 in either a motor or generator mode in the knownmanner.

The dc sides of the phase drive circuits 201 to 204 are connected toboth a first dc bus 206 and a second dc bus 207. A set of eightelectrical contactors 208 to 215 between the dc side of the phase drivecircuits 201 to 204, and the first dc bus 206 and the second dc bus 207,providing reconfigurable connection and isolation therebetween. In thepresent configuration, the contactors 208 to 215 operate under thecontrol of controller 205. In a non-faulted mode of operation, phases ϕAand ϕC of the motor-generator 112 are connected to the first dc bus 206,and phases ϕB and ϕD of the motor-generator 112 are connected to thesecond dc bus 207. This may be achieved by controller 205 settingcontactors 208, 211, 212, and 215 to a closed condition, and settingcontactors 209, 210, 213, and 214 to an open condition.

As will be described with reference to FIGS. 3 and 4, this configurationof the system permits each phase ϕA, ϕB, ϕC, and ϕD to remain isolatedduring fault-free operation.

Operation of the controller 205 in response to fault conditions will bedescribed further with reference to FIGS. 10 to 12.

It will of course be appreciated that other configurations may beemployed with reduced contactor count, for example utilising fuses toisolated faulted elements.

FIG. 3

The winding configuration of the motor-generator 112 is shown inschematic form in FIG. 3.

The motor-generator 112 comprises a rotor 301 located interior to astator 302. The rotor 301 is of permanent magnet configuration, and inthis example has 14 poles, i.e. the pole pair number p=7.

The stator 302 is configured as an alternate-wound stator, and in thisexample has sixteen teeth defining sixteen slots, i.e. the slot numberNs=16. Eight evenly-spaced coils 303 to 310 are located on alternateteeth such that there is one coil side per slot—this arrangement mayalso be referred to as a modular winding. This provides physical,thermal, electrical, and magnetic isolation between the coils whichprovides fault tolerance. In a specific embodiment, the coils 303 to 310are configured as precision coils, i.e. coils formed identically suchthat each turn occupies a specific, predefined location on the coil.

In an embodiment, short circuits are accommodated by configuring eachcoil to have a per-unit inductance of about unity. In an embodiment, theper-unit inductance is from 1.1 to 1.4, which may be accommodated byproviding appropriate insulation and cooling capacity, without imposingas high losses during normal operation as with a strict one-per-unitinductance design. In a specific embodiment, the per-unit inductance is1.25. In this way, the short circuit current is limited to 1.25 normaloperational values.

In an embodiment a non-overlapping winding approach is adopted whichresults in smaller end-windings which improves efficiency.

In the present example, the selection of values for p and Ns result in afractional ratio of slot number and pole number (8/7), which results inlow cogging torque. Further, given the number of poles 2p and number ofslots Ns differ by only 2, the slot-pitch is nearly the same as thepole-pitch which improves flux-linkage.

Forming part of the electric drive system of FIG. 2, the motor-generator112 is configured as a four-phase machine. As noted previously, thestator 302 comprises eight coils 303 to 310. In the present embodiment,the motor-generator is configured as a duplex four-phase machine, inwhich coil pairs separated by 180 degrees form part of the same phase.The individual coils in the present example are connected in parallelwith the respective phase drive circuit, although it is envisaged thatthey may also be connected in series. The choice may be made upon, forexample, the ability to run cables and/or the installation spaceenvelope for terminations, etc.

Thus, in the present example, coils 303 and 307 form a coil pairseparated by 180 degrees. Both coils form part of phase ϕA, with coil302 being labelled ϕA1, and coil 307 being labelled ϕA2. Similar angularseparation, nomenclature and labelling applies to the other coils. Thus,it may be seen that phase ϕB is separated by +45 degrees from OA, phaseϕB is separated by +90 degrees from ϕA, and phase ϕD is separated by+135 degrees from ϕA.

The approach adopted herein of pairing coils together means that themachine is mechanically balanced, in particular during a fault conditionwhen one or more phases may be disabled.

In the present embodiment, the rotor 301 comprises 14 permanent magnets311. In a specific embodiment, the magnets 311 are arranged in a Halbacharray. Halbach array rotors produce a larger overall flux and thereforepower capability for a given amount of magnetic material, and also tendto produce a near-sinusoidal air-gap flux distribution which leads tosmooth operation. In the present embodiment, the magnets 311 aresamarium cobalt magnets, which material is selected due to itsrelatively high temperature capability. It is envisaged that ifsufficient cooling capacity can be guaranteed, then neodymium magnetsmay be used instead. It will be appreciated that other known suitablepermanent magnet materials may be substituted.

FIGS. 4 & 5

Radial and axial cross sections of an embodiment of the motor-generator112 are shown in FIG. 4 (section I-I) and FIG. 5 (section II-II)respectively.

The stator 302 comprises a yoke 401 on which the coils 303 to 310 aremounted. In the present embodiment, each slot in the stator 302 isdefined by a wound tooth carrying a coil, and an unwound tooth notcarrying a coil.

In the present embodiment, the yoke 401 is formed of a lamination stackof the known type. In a specific embodiment, the laminations are ironcobalt laminations, although silicon iron or any other suitablelamination material may be used as an alternative. In the presentembodiment, the laminations are 0.2 millimetres in thickness, althoughit will be appreciated that other suitable thicknesses may be chosen tobalance losses and manufacturing complexities, etc.

The rotor 301 comprises a hollow outer magnet carrier 503 in which thepermanent magnets 311 are retained by banding such as carbon fibre orsimilar. In the present embodiment, the ratio of the radius of the rotor301 to the radial depth of the permanent magnets 311 is from 0.2 to 0.3.In a specific embodiment this ratio is 0.25. This provides the requisitepower rating despite the high inductance configuration for faulttolerance. Furthermore, selection of ratios within the aforesaid rangereduces the possibility of de-magnetisation due to high temperatureoperation, such as during a short circuit fault condition.

In a specific embodiment, the hollow outer magnet carrier 503 includesgrooves underneath the permanent magnets 311 to reduce eddy currentlosses. In this way, the hollow outer magnet carrier 503 may bemanufactured as a single-piece component, rather than necessitating asegmented design.

The rotor further comprises an inner component 504 (FIG. 8B) which inthis embodiment is press fit into the outer magnet carrier 503 intowhich a driveshaft 505 is engaged by splines (not shown).

As will be described further with reference to FIG. 7, in the presentexample the motor-generator 112 incorporates various features tofacilitate passage of coolant through the machine to effect removal ofheat. A fluid inlet 506 is provided at the non-drive end of the rotor301, and a fluid outlet 507 is provided at a base of the motor-generator112. Referring to FIG. 5, in a specific embodiment, the coolant is anoil, which may thereby also be used for lubrication of the rotorbearings 501 and 502.

FIG. 6

A magnified view of four slots of the stator 302 is shown in FIG. 5.

As described previously, the yoke 401 has a plurality of teeth definedthereon. In the present example, teeth are divided into a set of woundteeth 601 carrying coils, for example coil 303, and a set of unwoundteeth 602.

In the present embodiment, the width of wound teeth 601 is greater thanthe width of unwound teeth 602. This increases the degree offlux-linkage, as the coil-pitch tends to the same value as thepole-pitch. In a specific embodiment, the width of wound teeth is twicethe width of unwound teeth.

Furthermore, in the present embodiment, the width of each tooth at itsroot W_(R) in a circumferential sense is greater than or equal to itswidth at the tip WT. The teeth therefore adopt a trapezoidalcircumferential profile. It has been found that this leads to anadvantageous reduction in torque ripple. Further, the trapezoidalcircumferential profile maintains a constant flux density along eachtooth's radial extent, which prevents localised saturation that maylimit performance under high load.

In the present embodiment, the geometric configuration of the teeth 601and 602 is such that the slots are substantially trapezoidal in crosssection.

In the present embodiment, the stator slot ratio, which is the ratio ofthe stator slot inside diameter Ø_(I) to the stator slot outer diameterØ_(O), is from 0.6 to 0.7. It has been found that adopting values inthis range optimises the output torque for a given overall machinediameter. In a specific embodiment, the stator slot ratio is from 0.62to 0.66. In a more specific embodiment, the stator slot ratio is 0.64.

As shown in the Figure, the coils such as coil 303 are formed from aplurality of turns of insulated conductor. In the present embodiment,the conductor is a transposed conductor. Transposed conductors aremulti-strand conductors in which each strand is insulated, and istransposed in order to occupy each possible position along a specificlength. The transposition of the strands may be continuous, discrete, orrandom. In this way, when the conductor is exposed to a magnetic field,each strand will on average link with the same number of flux lines asevery other strand, thus dividing current equally among the strands. Thestrands are of small enough diameter that little skin effect can occur,thereby reducing losses due to induced eddy currents caused by therotating rotor field.

In a specific embodiment, the transposed conductor is a litz conductor.Litz conductors are a particular type of transposed conductor in whichstrands of round cross-section are transposed continuously along thecable length. It will be appreciated however that other transposedconductors may be used instead, such as Roebel conductors which userectangular strands transposed at discrete intervals.

In the present embodiment, the coils are configured to provide a slotfill-factor greater than zero and less than unity.

In a specific embodiment, the slot fill-factor is from 0.22 to 0.28. Ina more specific embodiment, the slot fill-factor is 0.26. In this way,when mounted on a wound tooth 601, a void 603 is formed in each slotadjacent the unwound tooth 602.

In the present example, the coils are formed into substantiallyparallelogram-shaped cross section. Thus, in the present example, thevoids 603 are triangular in cross-section, with their size beingdetermined by the tooth size and the coil size.

As will be described further with reference to FIGS. 7A and 7B, thevoids 603 the voids formed in the slots may act as axial coolingchannels for a flow of coolant therethrough to impingement cool thecoils 303 to 310.

FIGS. 7, 8A, 8B & 8C

As described previously, in the present embodiment the motor-generator112 comprises a coolant to facilitate cooling of the rotor and thestator. A diagram of the flow of coolant through the motor-generator 112is shown in FIG. 7.

Coolant, which as described previously may be an oil, but could insteadbe another suitable cooling fluid such as glycol etc., enters via inlet506. The fluid inlet 506 leads to a radial impeller 701 mounted to theinner component 504 at the non-drive end of the motor-generator 112.

Referring now to FIG. 8A, which is an end-on view of the radial impeller701, rotation of the rotor 501 and the impeller 701 thereby results inthe coolant being diverted radially in the direction of arrows R. Thisis accompanied by a centrifugal pressure rise.

Referring briefly to FIG. 5, it will be seen that coolant in the radialimpeller 701 is contained by the outer magnet carrier 503, and thus is,as shown in FIG. 7, forced into a helical path.

A side view of the inner component 504 is shown in FIG. 8B, andillustrates a helical channel 801 formed thereon. The helical channel801, together with the inner surface of the outer magnet carrier 503,form a helical fluid conduit underneath the permanent magnets 311. Inoperation, the helical fluid conduit acts as an Archimedes screw to drawcoolant along the underside of the magnets 311 to remove heat therefrom.

After travelling through the helical fluid conduit to the drive end ofthe motor-generator 112, the coolant then emerges in a collector (508,FIG. 5) and is diverted to travel through the axial fluid channels inthe stator slots.

The path the coolant takes in the present embodiment is shown in FIG.8C, which is a developed plan view of the circumferential sectionIII-III of FIG. 4. As shown in the Figure, the coolant follows aserpentine path through the axial fluid channels in the stator slots,facilitated by interconnecting ducts 802. A similar configuration isemployed for the other 180 degrees of the stator. Eventually afterpassing through the axial fluid channels in the stator slots, thecoolant exits the motor-generator 112 via the fluid output 507.

In an alternative embodiment, the coolant may alternatively be deliveredfrom the helical fluid conduit into a first manifold at the drive end ofthe motor-generator 112, whereupon it may be delivered in parallel to,for example, all of the axial fluid channels. A second manifold may thenbe arranged to collect the coolant at the non-drive end of themotor-generator 112 and deliver it to a fluid outlet at the same end. Itwill be appreciated that alternatively a first manifold at the drive endmay be arranged to pass coolant through a portion of the axial fluidchannels, with a ducts returning the coolant through the remaining axialfluid channels to a second manifold at the drive end for consolidationand output via the fluid outlet 507. Indeed, it is envisaged that acombination of such or other approaches could be adopted.

It is envisaged that the motor-generator 112 may use the same oil systemas the rest of the engine 101, and thus the coolant would be aircraftengine oil. It should be noted that a significant advantage of thecoolant circuit described with reference to FIG. 7 is that there is noflooding of the airgap between the rotor 301 and the stator 302 whichsignificantly reduces oil heating and oil churn.

FIGS. 9A & 9B

In the present embodiment, the teeth 601 and 602 on the yoke 401 are ofstraight-tooth configuration, nominally providing open slots. In thisway, the coils 303 to 310 may be formed off-iron prior to placing themon the yoke 401. Alternatively, the coils 303 to 310 may be woundon-iron, and as will be appreciated by those skilled in the art, theprocess of doing so is less complex due to the open slots.

An approach for sealing the voids 603 to prevent leakage of coolant istherefore shown in FIGS. 9A and 9B. In the present embodiment sealingmembers 901 are provided to seal the voids 603. The sealing members 901are engaged with the tips of adjacent teeth to circumferentially andaxially seal the voids 603.

In normal electric machine designs, it is optimal to reduce the leakageflux as this does not contribute to developing torque. However, aspreviously described, for fault tolerance it is desirable to achieve aper-unit inductance close to unity. An open slot design is not normallyconducive to this requirement, as tooth-tip leakage is typically a largecomponent of the total leakage flux in permanent magnet electricalmachines.

Thus, the sealing members 901 are comprised of a magnetic material toprovide a flux path between the tooth tips to increase the leakage flux.It will be appreciated that the radial thickness and the magneticpermeability of the sealing members 901 may be selected to achieve therequired leakage flux and thus obtain the desired inductance. In thepresent embodiment, the sealing members are formed of a soft magneticcomposite (SMC) material consisting of a glass fibre epoxy having ironpowder distributed therethrough. It is contemplated though thatalternative materials with a distributed air gap (i.e. magnetic materialparticles in a filler) such as molybdenum permalloy powder or similarmay be used.

The leakage flux may be controlled through variation of the permeabilityof the sealing members 901. In this way, the per-unit inductance, andtherefore short-circuit current characteristics may be controlled byappropriate selection of the sealing members' material magneticproperties.

In an embodiment, the saturation flux density of the sealing members 901is different to that of the rest of the stator yoke 401. In this way theelectric machine's characteristics differ during the motor mode ofoperation compared to the generating mode of operation. In particular,in the motor mode of operation it is possible to saturate the sealingmembers 901 which in turn reduces the overall inductance of each coil.This in turn causes the power factor to change, reducing the drivevoltage to achieve the same current at a given frequency. During thegenerating mode of operation, however, the only flux is that produced bythe magnets 311 and thus the saturation of the sealing members andattendant reduction in inductance does not occur.

A further advantage of the use of the sealing members 901 in effectivelyforming closed slots is that the rotor 301 is shielded from any strayelectric fields that may exist during switching of the coils 303 to 310by the phase drive circuits 201 to 204. In operation, stray fields causecharge to be built up on the rotor, especially with higher switchingspeeds and the higher the power output. This built-up charge couldotherwise, unchecked, subsequently discharge through the oil film of thebearings 501 and 502 causing electrical erosion and reducing bearinglife.

As shown in FIG. 9A, the tooth tips of both the wound teeth 601 and theunwound teeth 602 comprise axial grooves 902. Referring now to FIG. 9B,which is an isometric view of a sealing member 901, corresponding axialribs 903 are provided on each side for cooperation with the axialgrooves 902 so as to retain the sealing members in position.

In order therefore assemble the present embodiment of themotor-generator 112, it is possible to follow the following process.After obtaining the yoke 401, the coils 303 to 310 are placed thereon.As described previously, they may either be formed off-iron, and simplyplaced onto the wound teeth 601, or alternatively they may be wounddirectly on the wound teeth 601 using known techniques such as hairpinwinding or similar. Sealing members 901 are then engaged with the teeth601 and 602 on the yoke 401. In the present embodiment in which theteeth feature axial grooves 902 and the sealing members 901 featureaxial ribs 903, this may be achieved by sliding the sealing membersaxially into engagement.

In an embodiment, the process may further comprise a performing a vacuumpressure impregnation (VPI) procedure to seal the sealing members 901 tothe yoke 401. As will be appreciated by those skilled in the art, thisinvolves applying a varnish or a resin to the inner surface of thestator 302 following engagement of the sealing members 901 with theteeth 601 and 602, after which the assembly is rotated whilst subjectedto a vacuum followed by a high pressure atmosphere. This fills any gapsin the assembly, and removes the need for a stator sleeve to preventleakage of coolant into the airgap.

It will be appreciated that the cooling configuration and use of sealingmembers may be applied to electric machines with different numbers ofteeth and coils, for example to electric machines with four or moreteeth and alternate wound stators, i.e. n/2 coils.

FIG. 10

As described previously with reference to FIG. 2, the controller 205controls the operation of both the phase drives 201 to 204 and the stateof the contactors 208 to 215. In this way, the electric drive system mayrespond to both faults in the motor-generator 112 and on the first dcbus 206 and the second dc bus 207.

Steps carried out by the controller 205 in operation are set out in FIG.10. At step 1001, the controller 205 begins standard non-faultedoperation which in the present embodiment is under direction from theEEC 113, i.e. either operating as a motor to initiate start of theengine 101 or otherwise add torque, or operating as a generator tosupply the first dc bus 206 and the second dc bus 207.

After some time, one of two types of fault may occur. In somecircumstances, a phase fault 1002 may occur, due to an issue with one ormore of the phase drives 201 to 204, one of the phases in themotor-generator 112 itself, or possibly with the conductors between aphase drive and the motor-generator.

Such faults may be sensed on the basis of a measurement of any ofcurrent flow or voltage of each phase. For example, the fault may besensed using one or more of overcurrent protection, ground (earth) faultprotection, unit (or differential) protection and negative phasesequence protection. The fault may be sensed by one or more of a currenttransformer and a voltage transformer or digital equivalents.

In response to the identification of such a fault and as such the lossof operation of one of the phases, the controller 205 proceeds to step1003 in which the phase fault is mitigated. Procedures carried outduring step 1003 will be described with reference to FIG. 11.

Alternatively, a bus fault 1004 occurs on one of the first dc bus 206and the second dc bus 207. In response to this, the controller 205invokes procedures to reconfigure connections to the dc busses at step1005. Procedures carried out during step 1003 will be described withreference to FIG. 11.

The functionality in the controller 205 thereby allows the electricdrive system to continue operation in the presence of a single fault onthe ac side of the phase drive circuits 201 to 204, and a single faulton the dc side of the phase drive circuits 201 to 204. Even when havingentered a faulted mode of operation, the configuration of themotor-generator 112 is such that a further fault does not lead to ahazardous or catastrophic event. This is due to the modular phasewindings guaranteeing adequate independence of the phases. This isparticularly advantageous when the systems described herein are appliedin an aerospace environment, in the that an aircraft may be dispatchedwith a single fault in the electric drive system, which allowssufficient time for repairs to be organised and the aircraft to returnto a service location.

FIG. 11

Procedures carried out during step 1003 when operating in a motor modeof operation are set out in FIG. 11.

Following occurrence of a phase fault 1002, at step 1101 the faultedphase is identified. At step 1102 a question is asked as to which phasehas experienced a fault.

If the faulted phase is either phase ϕA or ϕC, then control proceeds tostep 1103 in which both of the corresponding phase drive circuits aredisabled. In the present embodiment this would be phase drives 201 and203. It will therefore be appreciated why in the present embodiment itis particularly advantageous to employ the duplex winding schemedescribed previously with reference to FIG. 3, as in this faultcondition it is possible to retain mechanical balance when still onlyoperating with phases ϕB or ϕD.

If instead the faulted phase is either phase ϕB or ϕD, then controlproceeds from step 1102 to step 1104 in which both of these phase drivecircuits are disabled. In the present embodiment this would be phasedrives 202 and 204.

Similar procedures may be performed in a generator mode of operation inresponse to a phase fault. Following these procedures, then in anembodiment the remaining operational phases are connected with both dcbusses to maintain supply thereto.

FIG. 12

Procedures carried out during step 1005 are set out in FIG. 12.

Following occurrence of a bus fault 1004, at step 1201 the faulted busis identified. At step 1202 a question is asked as to which bus hasexperienced a fault.

If the faulted bus is first dc bus 206, then control proceeds to step1203 in which the controller 205 opens the contactors 208 and 212(corresponding to phase ϕA and phase ϕC respectively) and the first dcbus 206. At step 1204, the controller 205 closes the contactors 209 and213 (corresponding to phase ϕA and phase ϕC respectively) and the seconddc bus 206. This reconfiguration therefore supplies the second dc bus206 from all of phases ϕA, ϕB, ϕC, and ϕD.

If the faulted bus is second dc bus 207, then instead control proceedsfrom step 1202 to step 1205 in which the controller 205 opens thecontactors 211 and 215 (corresponding to phase ϕB and phase ϕDrespectively) and the second dc bus 207. At step 1204, the controller205 closes the contactors 210 and 212 (corresponding to phase ϕB andphase ϕD respectively) and the first dc bus 205. This reconfigurationtherefore supplies the first dc bus 205 from all of phases ϕA, ϕB, ϕC,and ϕD.

At some future point in time, the dc bus fault may clear, in which casecontroller 205 may reverse the operations described above to reconfigurethe electric drive system back to its normal mode of operation.

It should be noted that whilst the present embodiments have beendescribed with reference to a turbofan engine 101 for an aircraft, itwill be understood that the principles of the described electricalsystem and electric machine may be applied to other installations, forexample in a marine environment such as on a naval vessel powered by gasturbines, or in an energy production environment such as in a powerstation utilising natural gas fired gas turbines, or any other suitableapplication.

Furthermore, it is contemplated that the electrical system and electricmachine configurations described herein may be extended to facilitateconnection of rotary electric machines with other types of rotatingmachinery. For example, the rotary electric machines may be connectedwith other types of heat engines, for example internal combustionengines such as reciprocating or Wankel-type engines. Other types ofheat engines such as steam turbines operating according to the Rankinecycle may be connected.

Various examples have been described, each of which comprise variouscombinations of features. It will be appreciated by those skilled in theart that, except where clearly mutually exclusive, any of the featuresmay be employed separately or in combination with any other features andthe invention extends to and includes all combinations andsub-combinations of one or more features described herein.

1. A stator for a radial-flux rotary electric machine, comprising: ayoke with a plurality of teeth, each tooth extending from a root at theyoke to a tip, the width of each tooth at the root being greater than orequal to the width at the tip thereof to define open slots therebetween;a plurality of coils, each one of which is located on a respective oneof the plurality of teeth, wherein the coils are sized such that theslot fill factor of between zero and unity to define axial voids in theslots; a plurality of magnetic sealing members, each one of which isengaged with the tips of adjacent teeth to circumferentially and axiallyseal the axial voids in the slots for retaining an axial flow of coolanttherein.
 2. The stator of claim 1, in which the stator is analternate-wound stator having a number n 4 of teeth and n/2 coils, andin which each slot is defined by a wound tooth carrying a coil, and anunwound tooth not carrying a coil.
 3. The electric machine of claim 2,in which the width of wound teeth is greater than the width of unwoundteeth.
 4. The electric machine of claim 3, in which the width of woundteeth is twice the width of unwound teeth.
 5. The stator of claim 1,having 16 slots and four coil pairs, each coil pair forming part of oneof four independent electrical phases.
 6. The stator of claim 5, inwhich each coil pair comprises coils that are 180 mechanical degreesapart.
 7. The stator of claim 1, in which the tip of each tooth includesaxial grooves for receiving axial ribs on the sealing members.
 8. Thestator of claim 1, in which the coils are formed so as to have asubstantially parallelogram-shaped cross section, such that the axialvoids are of triangular cross section.
 9. The electric machine of claim1, having a slot fill-factor of 0.22 to 0.28.
 10. The electric machineof claim 1, in which the ratio of the stator inner diameter to thestator outer diameter is from 0.6 to 0.7.
 11. The electric machine ofclaim 10, in which the ratio of the stator inner diameter to the statorouter diameter is from 0.63 to 0.67.
 12. The electric machine of claim1, in which each coil has a per-unit inductance of from 1.1 to 1.4. 13.The electric machine of claim 1, in which a saturation flux density ofthe sealing members is different to a saturation flux density of theyoke.
 14. A method of assembling a stator for a radial flux rotaryelectric machine, comprising: obtaining a stator yoke with a pluralityof teeth, each of which extends from a root at the yoke to a tip, thewidth of each tooth at the root being greater than or equal to the widthat the tip thereof to define open slots between adjacent teeth; for eachone at least a portion of the teeth, placing a coil on a tooth, whereinthe coil is sized such that the slot fill factor of between zero andunity to define an axial void in the slots either side of the tooth; foreach slot, engaging a magnetic sealing member with the tips of theadjacent teeth to circumferentially and axially seal the axial void inthe slot for retaining an axial flow of coolant therein.
 15. The methodof claim 14, further comprising a vacuum pressure impregnation procedureafter the engaging step to seal the sealing members to the stator yoke.16. The method of claim 14, further comprising forming the coilsoff-iron prior to placing them on the stator yoke.
 17. The method ofclaim 14, in which the placing step comprises winding the coils on thestator yoke.